The ability to identify proteins and determine their chemical structures has become central to the life sciences. The amino acid sequence of proteins provides a link between proteins and their coding genes via the genetic code, and, in principle, a link between cell physiology and genetics. The identification of proteins provides a window into complex cellular regulatory networks.
Ion trap mass spectrometers are among the most widely used platforms for molecular analysis—spanning natural products, to pharmaceuticals, to biologics such as proteins. Most mass spectrometer-based experiments begin with the isolation of a group of compounds from a set of samples through some sort of extraction technique, e.g., proteins from tissues, cell lysates, or fluids followed by proteolytic digestion of those proteins into peptides (i.e., bottom-up proteomics). Frequently, but not necessarily, the mass spectrometers are then coupled with some form of separations, e.g., electrophoretic or chromatographic. Over the course of just a few hours, mass spectral instruments can autonomously interrogate tens of thousands of molecular species via tandem mass spectrometry.
Quantitative analysis in chemistry is the determination of the absolute or relative abundance of one, several, or all particular substance(s) present in a sample. For biological samples, quantitative analysis performed via mass spectrometry can determine the relative abundance of peptides and proteins. The accepted methodology for performing mass spectrometric quantitation is accomplished using a mass spectrometer capable of MS/MS fragmentation (i.e., triple quadropole or ion trap). The quantitation process can involve isobaric tagging of peptide precursors, which when combined with post-acquisition software, provides the relative abundance of peptides. However, when a peptide precursor is selected for tandem mass spectrometry, there are often interfering species with similar mass-to-charge ratios that are co-isolated and subjected to activation. These species are often other isobarically tagged peptides with different relative quantitation, which therefore disturb the quantitative measurement of the peptide of interest.
Isobaric labeling is an important quantitative method as it allows for multiplexing and is directly applicable to clinical samples. A significant source of error, however, occurs when another eluting peptide ion has a m/z value that is very near that of the selected precursor (˜50%, in our hands). The result is the isolation of both species, which are consequently co-dissociated, to produce a composite MS/MS spectrum. The resulting reporter ion ratios do not accurately reflect the relative abundances of either peptide; limiting both the precision and dynamic range of quantitation, as the median peptide ratio is close to 1:1.
The increasing popularity of iTRAQ for quantitative proteomics applications has spurred increased efforts to evaluate its relevance, accuracy, and precision for biological interpretation. Recently, some researchers have begun to assess the accuracy and precision of iTRAQ quantification as well as drawbacks which hinder the applicability and attainable dynamic range of iTRAQ. Some results suggest that crosstalk between interfering factors can result in underestimations. [Ow et al., “iTRAQ Underestimation in Simple and Complex Mixtures: ‘The Good, the Bad and the Ugly’”, Journal of Proteome Research, web publication Sep. 16, 2009]. It is clear that there is tantalizing potential for iTRAQ and other protein labeling methods to provide accurate quantification spanning several orders of magnitude. This potential can be limited, however, by several factors. First, for example, the existence of isotopic impurities often requires correction of mass spectral data to provide accurate quantitation which currently requires the availability of accurate isotopic factors. Second, the interference of mixed MS/MS contribution occurring during precursor selection is a problem that is currently very difficult to minimize.
Protein identification technologies have rapidly matured such that constructing catalogs of the thousands of proteins comprised by a cell using mass spectrometry (MS) is now relatively straightforward [de Godoy, L. M. F. et al. Nature 455, 1251-1255 (2008); Swaney, D. L., Wenger, C. D. & Coon, J. J. J. Proteome Res. 9, 1323-1329 (2010)]. Knowing how the abundance of these molecules change under various circumstances is not [Ong, S. E. & Mann, M. Nat. Chem. Biol. 1, 252-262 (2005)]. Stable isotope labeling by amino acids in cell culture (SILAC) provides a means to make binary or ternary comparisons [Jiang, H. & English, A. M. J. Proteome Res. 1, 345-350 (2002); Ong, S. E. et al. Mol. Cell. Proteomics 1, 376-386 (2002)]. By interlacing these two- or three-way experiments, higher-order comparisons can be obtained [Olsen, J. V. et al. Sci. Signal. 3, ra3 (2010)]. Such large-scale multiplexed experiments are invaluable, as they (1) allow measurement of time-course experiments, (2) permit collection of biological replicates, and (3) enable direct comparison of transcriptomic and proteomic data.
Constructing this type of multi-faceted proteomics study, however, is an arduous undertaking and has only been accomplished in a handful of experiments by an even smaller group of researchers. The first impediment is the requirement to grow multiple groups of cells with various labels. And this step is actually less limiting than the second major obstacle: each binary or ternary set must be analyzed separately. When combined with the need for extensive pre-MS fractionation and technical replicates, a large-scale experiment via SILAC demands three to six months of constant instrument usage.
As proteomics moves away from discovery based experiments towards hypothesis driven analyses, the measure of success is no longer the absolute number of proteins identified, but the identification of specific proteins of interest. This trend is exemplified in the surge of interest in selected reaction monitoring (SRM) based protein assays—a powerful tandem mass spectrometry method that can be used to monitor target peptides within a complex protein digest. These experiments typically entail careful optimization of fragmentation parameters, tight regulation of liquid chromatography (LC) conditions, and complex method scheduling. To date, in every mass spectrometer based protein assay, the spectrometer has been a passive agent. The user instructs the instrument on how to survey the precursor ion population, which precursors to interrogate, and how to interrogate those precursors. Essentially, the instruments require careful and detailed administration by the user, requiring laborious methods to produce sensitive, specific, and reproducible results.